Wireless devices often transmit and receive voice and other media in a compressed digital format in order to reduce bandwidth. In particular, a wireless device may receive voice or other media from a user, digitize it, compress the digital signals in accordance with various algorithms, and then transmit the voice or other media in the compressed digital format. A wireless device that receives media in the compressed digital format decodes it in order to uncover the original signal. Wireless devices typically include vocoders to convert the voice or other media between analog and compressed digital formats. A number of different compressed digital formats are commonly used.
One example is the Enhanced Variable Rate Codec (“EVRC”), which supports voice transmission at a data rate of about 8 Kbps. EVRC is described in the TIA/EIA/IS-127-3 standard, “Enhanced Variable Rated Codec, Speech Service Option 3 for Wideband Spread Spectrum Digital Systems.” Another example is the Selectable Mode Vocoder (“SMV”), which supports voice transmission at a data rate of about 4 Kbps. SMV is described in the TIA/EIA/IS-893 standard, “Selectable Mode Vocoder Service Option for Wideband Spread Spectrum Communication Systems.” SMV is a higher compression format than EVRC as it only requires about half the bandwidth of EVRC. Wireless devices that support the SMV format are typically backward compatible and also support the less efficient EVRC format.
In contrast, circuit-switched telephone networks, such as the public switched telephone network (“PSTN”), typically carry voice or media in an uncompressed digital format, such as a pulse code modulation (“PCM”) format. For example, in a typical PCM format, a voice signal is sampled at a rate of 8000 times a second and digitized with an 8-bit resolution, resulting in a 64 Kbps signal. Trunks used in the PSTN are often able to carry many of these 64 Kbps signals at a time in separate channels. For example, a T-1 trunk can carry 24 of these 64 Kbps channels.
Wireless telecommunications systems often use transcoders to convert between the compressed digital formats used by wireless devices and the PCM or other format used by circuit-switched telephone networks. However, using transcoders can result in inefficiencies. For example, in a call that involves two wireless devices, the voice or other media exchanged in the call may be transcoded twice. One transcoder may convert the media from the transmitting wireless device to PCM format for transmission through the PSTN, and another transcoder may convert the media from the PCM format to the compressed media format for transmission to the receiving wireless device. Using transcoders “in tandem” is often undesirable, because each transcoder can add delay to the call and can degrade the quality of the voice or other media.
To address these concerns, various proposals have been made to provide “tandem free operation” (“TFO”) in calls between wireless devices in which the transcoders are bypassed and the PSTN carries media in the compressed digital format without transcoding. One such method is described in 3rd Generation Partnership Project 2, “CDMA Tandem Free Operation,” 3GPP2 A. S0004-B v2.0 (Aug. 5, 2002). In this method, an inband signaling protocol is used to test if the call is a mobile-to-mobile call, if the wireless devices use the same compressed digital format, and if the systems at both end of the call support tandem free operation. If these tests are successful, the inband signaling protocol establishes a “transparent” digital channel through the PSTN, and the transcoders are bypassed. The media is then carried through the PSTN in the original compressed digital format, which is uses 16 Kbps “TFO frames” that are mapped onto the two least significant bits in a 64 Kbps PCM frame.
While a call might be carried over a circuit-switched network, the call might alternatively be carried over a packet switched network. For example, two wireless devices might be served by respective access points with the access points in turn communicatively coupled via a packet-switched network rather than the PSTN. In calls between the wireless devices where the wireless devices both support the same compressed digital format, the transcoders might similarly be bypassed so that the voice or media is then carried through the packet-switched network in the original compressed digital format to provide “transcoder free operation” for the call. Vocoder bypass is generally used to refer to bypassing transcoders in calls between wireless devices, with tandem-free operation and transcoder-free operation referring to the more specific cases when the calls are completed over circuit-switched and packet-switched networks respectively.
These vocoder bypass approaches still have disadvantages. For example, a first wireless device might include a higher compression vocoder (e.g., SMV), while a second wireless device might support a lower compression vocoder (e.g., EVRC). In negotiating to provide vocoder bypass, the wireless devices might both default to using the less efficient vocoder. In this case, the first wireless device would not then be able to use its more efficient vocoder, thereby using a larger amount of bandwidth on its air interface with an access point. Accordingly, there is still a need to provide efficient systems and methods for vocoder bypass.